Reciprocating Steam Engine V.S. Compound Steam Engine, Any Difference?

[Steamchick]

The best engine is one you can make a good job of finishing. Many start making "good" engines, but don't finish the job, so end up with disappointing volumes of scrap.
I buy some of the "scrap" - and occasionally make something useful from it. Sometimes it is just scrap.
But a finished engine that works is always a good engine. (But the best engines when worn-out are simply scrap!).
Start simple - enjoy success - grow bigger and happy.
(I enjoyed "simple" - so stayed there!).
K2

 

[Andy Munns]

hello Andy, I have one more question about the steam engine. how to tell which engine is better as there are so many choices.

Perhaps depends on the application. A single-cylinder engine or a compound will not start without the 'engineer' noting where the crank is and reversing the gear or positioning the crank manually.

Twin simple with cranks at 90 degrees will self-start and includes locomotives, winding engines, winches and steering engines. You don't really need a 3 cylinder double-acting simple and these were very rare, perhaps only seen on steel rolling mill engines. Normally the cost of the third and duplicate cylinder was not warranted.

So for display, probably does not matter what you have, therefore indulge your personal choice. For powering a locomotive or winch they were almost always twin simple. For a model boat with RC, the reverse will be something like a switch valve or Stephenson reverse, and as you can't get to the middle of the pond to flick the prop, a twin simple is likely mandatory. A 3 cylinder simple will do the same job, but with a cost and length penalty.

The model compounds and triples are lovely, but out in the pond where reverse is wanted, twin simples are needed.

 

[Charles Lamont]

You don't really need a 3 cylinder double-acting simple and these were very rare, perhaps only seen on steel rolling mill engines.

And larger railway locomotives, where 3 cylinders allowed more power without oversize cylinders hitting the edges of station platforms, improved starting, and reduced track damage caused by out-of-balance forces (known as 'hammer-blow').

Curiously, very few 4-cylinder locos had cranks at 45°. The vast majority had cranks at 90°, with two cylinders on opposite phase. This meant that only two sets of valve-gear were needed, working one valve directly, and the other with a rocker.

 

[JLaning427]

Putting the throws at 45° would really serve no purpose in a 4 cyl double acting engine. I can't see any real gain to be had, and I bet it would be harder to make and balance. Might end up with a slightly smoother torque curve.

James

 

[Andy Munns]

Yes - Not sure about the crank angles on these locos - Certainly rare. Then also, IC 4 cylinder engines balance out the rocking couples with a flat plane crankshaft.

At sea, the last gasp of steam saw 4 cylinder triples and quadruples with Yarrow, Schlick and Tweedie crank angles

There were also 4 cylinder Lentz double compounds with HP cranks at 90 degrees and 180 degree crank Woolf style compounding to LPs.

 

[Charles Lamont]

Putting the throws at 45° would really serve no purpose in a 4 cyl double acting engine. I can't see any real gain to be had, and I bet it would be harder to make and balance. Might end up with a slightly smoother torque curve.

That was usually the point. On the North Staffordshire railway Hookham built an odd-looking experimental 4-cylinder 0-6-0 tank engine with 45° cranks. He wanted to see if it would improve acceleration of suburban stopping trains. I think the conclusion was that it was not worth the extra complication.

 

[deverett]

My railway knowledge is scant, but I seem to remember that the 4 cylinder Southern Railway 'Lord Nelson' class designed by R.E.L. Maunsell had cranks set at 135 degrees to reduce hammer blow.
Dave
The Emerald Isle

 

[Steamchick]

The text books on locomotive design have fairly easy calculations on balance v hammer blow. BR. used to have one limit, but having cleared the tracks of steam in the early 1960s, I understand the hammer blow limit was reduced soon thereafter, as the diesels and electrics were easier on tracks, so they saved money in rails and track beds accordingly. Roll on 50 years, and I understand Tornado was built to the new spec for hammer blow, (wheel balance factor) and was tested to be well within the requirement. The anecdote from those tests suggests the accompanying diesel loco measured worse than Tornado on the approval measurement tests!
Hence hammer blow is not simply a result of crank configuration, but a complex problem of configuration and balance factors..... (it was there as an approved braking system while Tornado was being tested).
But I haven't read the text books for many years, so please correct me if I am speaking from the wrong end of my alimentary canal....
Cheers,
K2